Term long-run average cost curve Definition: A curve depicting the per unit cost of producing a good or service in the long run when all inputs are variable. The long-run average cost curve (usually abbreviated LRAC) can be derived in two ways. On is to plot long-run average cost, which is, long-run total cost divided by the quantity of output produced. at different output levels. The more common method, however, is as an envelope of an infinite number of short-run average total cost curves. Such an envelope is base on identifying the point on each short-run average total cost curve that provides the lowest possible average cost for each quantity of output. The long-run average cost curve is U-shaped, reflecting economies of scale (or increasing returns to scale) when negatively-sloped and diseconomies of scale (or decreasing returns to scale) when positively sloped. The minimum point (or range) on the LRAC curve is the minimum efficient scale.